The present invention is directed to improvements to medical ultrasonic imaging systems. In particular, control of transmit power is provided.
The Food and Drug Administration (FDA) limits the transmit power used by medical imaging systems. Most ultrasound systems use a transmit power close to this maximum as a default in those cases where the system's own limitations such as maximum peak voltage or maximum total power are not the limiting factors. However, the FDA desires, for every patient and imaging plane being scanned, the transmit power to be reduced to as low as reasonably achievable (ALARA) levels. But, to assure satisfactory signal-to-noise ratios (SNR), the FDA left any reduction in transmit power to the user to control.
The main sources of electronic noise in an ultrasound system are thermal and quantization noise in the receiver. The analog components of the receiver generate the thermal noise, and the analog-to-digital converters generate the quantization noise. Therefore, the electronic noise is generally independent of the transmit power. The received signal level is proportional to the peak transmit voltage for imaging at a fundamental transmit frequency, and the square of the peak transmitter voltage for imaging at a second harmonic of the transmit frequency.
On some ultrasound imaging platforms, the overall system gain compensates for the transmitter power. As the power management process of the ultrasound system readjusts the transmit power level in response to a change in the imaging parameters, or as the user manually reduces the power level, the display brightness is preserved in high SNR areas of the image. On such systems, the noise and not the signal level varies with the power level. But if, even after the power is reduced the noise level remains below a threshold based on the display dynamic range and the system gain, no or few visible changes result in the display image. The noise in the low SNR areas of the image are still mapped to the lowest display gray level, e.g., black. However, such system may not use transmit powers that are as low as reasonably achievable.